In a study of the potential prophylactic effects of long-term conditioning to various environments on the susceptibility of mice to decompression sickness it was found that treadmill exercise training enhanced decompression survival. Decompression depths of 7.6, 8.7, or 9.9 Atmospheres Absolute (pO2 equals 0.5 ATA) with N2 were used. Exposure to cold (4 degrees C-4hr/d-28d) or hypoxia (PB379 Torr-4hr/d-28 d) or swim exercise (15 min/d in 31 degrees C water-28 d) prior to decompression induced either no change in decompression survival rate (cold or swim exercise) or reduced survival rate (hypoxia). Enhanced survival in the Treadmill Exercise Group is presumably due to lean body confirmation and superior cardiovascular and respiratory fitness, whereas increased bends incidence in the Hypoxia Group may be attributable to greater blood viscosity due to polycythemia and hemoconcentration.